I finished the Kingdom of Ehb map at level 64 for my solo character. What about the other two single player maps I have which are Legends of Aranna and Return to Arhok. If I start a new solo character in those maps, what level approximately will I be if I finish them and kill all mobs along the way?

Are they any singleplayer maps out there I can download that will let me finish them closer to level 100?

I don't think so.
Legends of Aranna is about as long as the Kingdom of Ehb.
Yesterhaven and Return to Arhok start you out at level 10 or 20, but are really short and you'll finish at level 40 or so.

I haven't looked at every map you can download of course, but to get to level 100 it would have to be really long or level you up way faster than the official maps.
I remember there was an ultima mod that had a relatively long campaign, but I never played that so I cannot comment on the length.

Your best bet is going solo on Legends of Aranna or Kingdom of Ehb instead of taking a party. If I remember correctly that will get you to around level 80.
You can also play the multiplayer map Utrean Peninsula, on the 3rd difficulty setting it has areas with levels above 106. Since this is balanced for multiplayer consider getting an XP multiplier, otherwise it will take a long time to get there.

Note that you can import a single player character into multiplayer, but not the other way around.

If you prefer to have a party maybe look at the legendary mod for Dungeons Siege 2.
https://www.moddb.com/mods/dungeon-siege-legendary-pack
(download from files, for some reason the big link on the main page is for an old version)
Altough I think playing through the original DS2 maps 3 times will get you to level 100 as well.